Possible states of chloride in the hydration of tricalcium silicate in the presence of calcium chloride V.S. RAIVlACHANDRAN (i)

Calcium chloride may be present in the free, adsorbed or interlayer state in hydrating tricalcium silicate. Attempts have been made to study these states to correlate some of the physical, chemical and mechanical properties.

Calcium chloride is a well-known accelerator in c o n c r e t e practice. Most published data, h o w e v e r , relate to its influence on the e n g i n e e r i n g p r o p e r t i e s of concrete rather than to u n d e r s t a n d i n g of the basic mechanism. Early w o r k e r s b e l i e v e d that the interaction of the C3A(1) p h a s e of cement with CaCI~ was responsible for acceleration and strength d e v e lopment. Only recently have studies r e c o g n i z e d the p r e d o m i n a n t role of CaC12 in the hydration of silicate phases of c e m e n t [1-21]. Several explanations h a v e b e e n offered for the action of CaCI 2. The possibility that a c o m p l e x calcium o x y c h l o r i d e h y d r a t e is formed, p r o m o t i n g hydration in s o m e way, was p r o p o s e d b y Candlot [22], Koyanagi [23], Kallauner [24], Kleinlogel [25] and Tenoutasse [26]. It should b e r e c o g n i z e d that in the system CaO-CaClz-H~O two oxychtorides of c o m position 3 CaO.CaC12.16H~O and CaO.CaC12.2H20 exist [27-30]. The f o r m e r is stable at CaCI 2 c o n c e n trations of 18 p e r cent or more, and the latter at 34 p e r cent or more. In actual practice the concentration of CaC12 u s e d is m u c h l o w e r than the a b o v e figures, and on these g r o u n d s the possibility of formation of calcium

o x y c h l o r i d e c o m p l e x e s has g e n e r a l l y b e e n discounted. In addition, application of techniques such as X-ray, d y n a m i c differential a n d conduction calorimetry, electron m i c r o s c o p y and chemical analysis has not r e v e a l e d the p r e s e n c e of such c o m p l e x e s in h y d r a t i n g cements [3, 4, 9, 11, 14, 3I, 32]. In the a b s e n c e of any e v i d e n c e of a c o m p l e x comp o u n d b e t w e e n Ca(OH)2 and CaC12 in h y d r a t i n g c e m e n t s it is s u g g e s t e d that CaCl~ acts catalytically [4, 10, 20, 24, 32]. The exact m e c h a n i s m t h r o u g h which this action takes place, h o w e v e r , is still o b s c u r e . Addition of CaC12 to a h y d r a t i n g c e m e n t is k n o w n to r e d u c e the alkalinity of the a q u e o u s phase. It is thus b e l i e v e d that b y a r e d u c e d pH the s y s t e m would attempt to c o m p e n s a t e b y liberating m o r e lime t h r o u g h i n c r e a s e d rate of hydrolysis of CzS [3, 4, 31, 33]. Acceleration can also b e b r o u g h t about in an e n v i r o n m e n t of higher pH values and it is doubtful w h e t h e r acceleration is b a s e d on pH effects only. A n y p r o p o s e d m e c h a n i s m should r e c o g n i z e that calcium chloride, in addition to modifying the h y d r a tion kinetics of CaS, affects chemical composition, physical and mechanical properties of the s y s t e m at various stages of hydration. These manifest themselves in terms of induction period, initial and final set, CaO/SiO~ ratio of the h y d r a t e d silicate, surface area, microstructure, pH of the a q u e o u s phase, shrinkage, strength and resistance to sulphate attack and freezing-thawing. It is e x t r e m e l y unlikely that any one m e c h a n i s m could explain all these effects,

(1) National Research Council of Canada, Division of Building Research.

(1) The following nomenclature used in cement chemistry will be followed where necessary : C = CaO, S = SiO 2, A = AI203 and H -- H20.

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a n d a c o m b i n a t i o n of factors m a y b e involved, d e p e n d i n g on the e x p e r i m e n t a l conditions a n d p e r i o d of hydration. In s t u d y i n g the kinetics of h y d r a t i o n of C3S in the p r e s e n c e of CaC12 b y t h e r m a l m e t h o d s , t h e r e was e v i d e n c e of v a r i o u s states of c h l o r i d e , i n c l u d i n g c o m p l e x e s [34]. This e v i d e n c e l e d to a n e w s e r i e s of e x p e r i m e n t s the results of which a r e now p r e s e n t e d with a d i s c u s s i o n of the p o s s i b l e mechanism.

EXPERIMENTAL Materials The s a m p l e of tricalcium silicate u s e d in the p r e s e n t w o r k was m a d e a v a i l a b l e b y the P o r t l a n d C e m e n t Association, U.S.A., a n d h a d the following c o m p o sition e x p r e s s e d as a p e r c e n t a g e i g n i t e d basis: Chemical CaO SiO 2 AI~_O3

= = =

F r e e Cao (ASTM) F r e e CaO (Franke)

= =

73.88 26.17 0.08 100.13 0.18 0.46

C3S

=

99.33

C~S C3A C a O (Franke)

= = =

0.00 0.21 0.46 100.00

Mineralogical

F i n e n e s s = Blaine 3310

sq

cm/g

C a l c i u m c h l o r i d e h e x a h y d r a t e of analytical r e a g e n t quality was u s e d as the a c c e l e r a t i n g admixture. "As the s o l i d is d e l i q u e s c e n t , solutions of r e q u i r e d conc e n t r a t i o n s c o u l d not b e p r e p a r e d d i r e c t l y b y w e i g h i n g a n d d i s s o l v i n g in water. A p p r o x i m a t e l y 15 p e r cent CaC12 solution was t h e r e f o r e p r e p a r e d a n d the e x a c t c o n c e n t r a t i o n d e t e r m i n e d b y the a r g e n t o m e t r i c m e t h o d . Dilutions w e r e m a d e to a n y r e q u i r e d concentration.

Sample Preparation H y d r a t i o n of C~S was s t u d i e d b y m i x i n g it with d o u b l e - d i s t i l l e d w a t e r at a water-silicate ratio of 0.5. H y d r a t i o n was c a r r i e d out in t i g h t l y - c o v e r e d p o l y e t h y l e n e c o n t a i n e r s r o t a t e d c o n t i n u o u s l y o v e r rollers. At s p e c i f i e d intervals, v a r y i n g b e t w e e n 15 minutes a n d 1 month, e a c h s a m p l e was g r o u n d , p l a c e d in a d e s i c c a t o r a n d continuously e v a c u a t e d for 24 hours, u s i n g l i q u i d air in the trap. C a r e was t a k e n t h r o u g h out to p r e v e n t contamination with CO 2. A similar m e t h o d was f o l l o w e d for the h y d r a t i o n e x p e r i m e n t s in the p r e s e n c e of different c o n c e n t r a tions of CaC12. The solution-silicate ( v o l u m e / w e i g h t ) ratio was k e p t at 0.5. This could b e a c h i e v e d with 1, 4 a n d 5 p e r cent CaCI~ (with r e s p e c t to C3S) b y a d d i n g 10 cc e a c h of 2, 8 a n d 10 p e r c e n t CaCI 2 solution, r e s p e c t i v e l y , to 20g of C3S. The r e a c t i o n was c a r r i e d out at a t e m p e r a t u r e of 70 :~ 1 ~

Analysis Differential t h e r m a l analysis (DTA) was c a r r i e d out u s i n g the Du Pont 900-Thermal A n a l y s e r . This

unit utilizes p l a t i n u m h o l d e r s a n d platinum vs platinumr h o d i u m (13 p e r cent) t h e r m o c o u p l e s w e r e u s e d for differential a n d s a m p l e temperature measurements. The r e f e r e n c e m a t e r i a l was i g n i t e d ~-A1203 a n d the rate of h e a t i n g was 20 ~ In e a c h run 50 m g of the s a m p l e was p a s s e d t h r o u g h a 100m e s h s i e v e a n d p a c k e d with m o d e r a t e p r e s s u r e . T h e r m o g r a m s w e r e o b t a i n e d in air, continuous vacuum, or in a continuous flow of n i t r o g e n at a p r e s s u r e of 1.5 in. The s e n s i t i v i t y of the differential t e m p e r a t u r e on the Y axis w a s 0.004 m V / i n , for most of the e x p e r i m e n t s , with s a m p l e t e m p e r a t u r e on the X axis at 2 m V / i n . C o l d j u n c t i o n was m a i n t a i n e d at 0 ~ with c r u s h e d ice. R e f r a c t o r y c u p s p l a c e d in the s t a n d a r d p l a t i n u m s a m p l e h o l d e r s w e r e u s e d in the e x p e r i m e n t s , e s p e c i a l l y t h o s e involving s a m p l e s with h i g h e r CaCI~ content. O t h e r w i s e , the s a m p l e t e n d e d to fuse to s o m e e x t e n t a n d stick to the c o n t a i n e r a n d the t h e r m o c o u p l e (and it w a s not e a s y to r e m o v e it). Many s a m p l e s w e r e r u n in d u p l i c a t e a n d the results s h o w e d g o o d r e p r o d u c i b i l i t y . Calcium h y d r o x i d e , f o r m e d at different p e r i o d s o h y d r a t i o n , was e s t i m a t e d b y d e t e r m i n i n g the e n d o t h e r m a l a r e a of d e h y d r a t i o n . T h e r m o g r a v i m e t r i c analysis (TGA) of the s a m p l e s was o b t a i n e d b y the s t a n d a r d Stanton t h e r m o b a l a n c e at a h e a t i n g rate of 10 ~ X - r a y diffraction results w e r e o b t a i n e d b y a H i l g e r d i f f r a c t o m e t e r u s i n g CuK~ s o u r c e . E x p e r i m e n t s w e r e also c a r r i e d out to d e t e r m i n e the c h l o r i d e content of solutions l e a c h e d with a b s o l u t e alcohol o r water. The m e t h o d c o n s i s t e d of a d d i n g 5 cc 10 p e r cent CaC1 z solution to 10g C3S in a p o l y e t h y l e n e container, rotating it o n r o l l e r s for different p e r i o d s , r e m o v i n g it a n d g r i n d i n g it in c o l d a b s o l u t e alcohol. The s a m p l e was c o n t i n u o u s l y w a s h e d o v e r a filter p a p e r with alcohol o r w a t e r a n d the l e a c h a t e c o l l e c t e d in a s t a n d a r d flask e n c l o s e d in a c h a m b e r . E a c h g r a m of h y d r a t e d s a m p l e was w a s h e d with a b o u t 100 cc alcohol o r w a t e r a n d this is r e f e r r e d to as l e a c h i n g in the following text. The s o l i d m a t e rial left on the filter p a p e r w a s d r i e d in v a c u u m for 24 h o u r s a n d s u b j e c t e d to D T A e x a m i n a t i o n in air, v a c u u m o r n i t r o g e n . Due p r e c a u t i o n was t a k e n to p r e v e n t c a r b o n a t i o n of the s a m p l e . The c h l o r i d e content in the l e a c h a t e was e s t i m a t e d b y the a r g e n t o m e t r i c m e t h o d , u s i n g s t a n d a r d solutions of s i l v e r nitrate a n d a m m o n i u m thiocyanate, with ferric alum as the i n d i c a t o r [26]. A b l a n k s e r i e s was also run b y l e a c h i n g p u r e C3S after h y d r a t i o n to c o r r e s p o n d i n g p e r i o d s .

RESULTS KND DISCUSSION G o o d c o r r e l a t i o n o f DTA a n d T G A results was o b t a i n e d for the estimation of Ca(OH) 2 at a n y s t a g e of h y d r a t i o n . C o m p a r i s o n of e s t i m a t e d Ca(OH) 2 a n d the rate of d i s a p p e a r a n c e of C3S i n d i c a t e d that with h i g h e r CaC12 content the C-S-H p r o d u c t h a d a h i g h e r C a O / S i O 2 ratio than that f o r m e d without CaCI 2. The a d d i t i o n of CaCI~ in a m o u n t s of 1 to 5 p e r c e n t i n c r e a s e d the r a t e of h y d r a t i o n of C3S p r o f o u n d l y , e s p e c i a l l y e a r l y in the e x p e r i m e n t . A c o n s i d e r a b l e a m o u n t of h y d r a t i o n within a few h o u r s must significantly influence e v e n the n a t u r e of h y d r a t i o n p r o d u c t s . Hence, an u n d e r s t a n d i n g of the h y d r a t i o n r e a c t i o n s in the e a r l i e r p e r i o d s s h o u l d h o l d the k e y to the effective action of CaC12 on the h y d r a t i o n of C3S. In a d d i t i o n to an intensification of c e r t a i n e n d o t h e r mal effects in the p r e s e n c e of CaCI 2, significant n e w d e v e l o p m e n t s a r e o b s e r v e d , viz., an e n d o t h e r m a l

V . S. R A M A C H A N D R A N

effect b e t w e e n 550 and 560 ~ one or two i n t e n s e e x o t h e r m i c effects in the t e m p e r a t u r e r a n g e 600 to 800 ~ an e n d o t h e r m a l p e a k of l a r g e m a g n i t u d e at 800 to 850 ~ d e p l e t i o n of p e a k s d u e to p h a s e transitions a n d an emergence of a n e w exothermal effect in the high t e m p e r a t u r e r e g i o n s . A further investigation was m a d e to e x a m i n e the p o s s i b l e c a u s e s of t h e s e t h e r m a l effects a n d their role in the a c c e l e r a t i n g action of CaCI~. Surface Complex mant Period

of Chloride

during

the

Dor-

The e n d o t h e r m i c effect at 550 to 560 ~ can b e o b s e r v e d w h e n C3S is p l a c e d in a CaC12 solution e v e n for a few minutes. This has not b e e n reported b e f o r e . The p o s s i b i l i t y that CaC12.6H20 in the f r e e state is r e s p o n s i b l e can b e d i s c o u n t e d b e c a u s e p u r e CaCI2.6H20 d o e s not exhibit an e n d o t h e r m i c effect at 550 to 560 ~ a n d the effect at 750 ~ r e p r e s e n t s fusion (fig. 1, c u r v e 1). Calcium c h l o r i d e is h i g h l y s o l u b l e in ethyl alcohol a n d this solvent was u s e d to l e a c h out f r e e c h l o r i d e f r o m the C3S h y d r a t e d for an h o u r in the p r e s e n c e of calcium chloride. In the l e a c h e d s a m p l e the e n d o t h e r m a l effect p e r s i s t s (fig. 1, c u r v e s 2 a n d 3), a n d this m e a n s that f r e e chlor i d e is not r e s p o n s i b l e for the e n d o t h e r m a I effect. L e a c h i n g of the s a m p l e with water, h o w e v e r , eliminates the effect (fig. 1, c u r v e 4). A n a d d i t i o n a l e n d o t h e r m a l effect also d e v e l o p s at 495 ~ o b v i o u s l y d u e to the formation of Ca(OH) 2 as a result of h y d r a tion of C.~S d u r i n g leaching.

I

f

i

< I

z e~

J

F

I

400

600

~

I

800 ~

C

TEMPERATURE

Fig. 1. - - Thermal curves of CaCl,.,. 6H,.~O and 3CaO. Si0: hydrated for 1 hour in presence of 5 ~ o CaCI2

(t) (2) (3) (4)

I00

I

I

9O o

8O

=

70

c~

60

x z

40

,z

30

I

o/

-

-/

I

I

I

I

I

LEACHED WITH ABS.

/

L__-L----4~

r ---~

ALCOHOL

so

c~:

I

20

LEACHED WITH WATER

/4

tO

4

6

8

10

12

14

16

PERIOD OF H Y D R A T I O N ,

18

20

22

24

168

HR

Fig. 2. - - Estimation of chloride content in hydrating C:,S in 5 % CaCI, solution.

i

o-

200

CaO with CaCI~. 6H.~O was s u b j e c t e d to DTA a n d the c u r v e s h o w e d a p e a k at 600 ~ As free calcium h y d r o x y c h l o r i d e is not e x p e c t e d to b e p r e s e n t u n d e r low CaCL c o n c e n t r a t i o n s p r e v a i l i n g at 1 hr, it is v e r y p r o b a b l e that the e n d o t h e r m a l effect at 550 to 560 ~ is the result of an a d s o r p t i o n c o m p l e x of c h l o r i d e a n d H~O f o r m e d on the h y d r a t i n g C3S s u r f a c e in the d o r m a n t p e r i o d . It is p o s s i b l e that this has a c o m p o sition similar to calcium h y d r o x y c h l o r i d e . TGA shows a small loss in w e i g h t c o r r e s p o n d i n g to the e n d e t h e r mal effect for this c o m p l e x ,

CaCh. 6H20 3Ca0.SiO2 hydrated for I hour in 5 ~o CaC12 2 leached with alcohol 9 leached with wvter.

C h e m i c a l analysis of C1- in alcohol o r w a t e r - l e a c h e d s a m p l e r e v e a l s that almost 100 p e r cent of the chloride is r e m o v e d b y water, w h e r e a s a l c o h o l e x t r a c t s o n l y a b o u t 94 p e r cent from a s a m p l e h y d r a t e d for 1 h r (fig. 2). Calcium h y d r o x y c h l o r i d e s h o w s an e n d o t h e r m i c effect at a b o u t 550 to 600 ~ [35, 36]. A p r e p a r a t i o n of calcium h y d r o x y c h l o r i d e f o r m e d b y r e a c t i n g

F o r m a t i o n of the a d s o r b e d c h l o r i d e c o m p l e x c o u l d not b e d e t e c t e d b e f o r e b y X - r a y o r calorim e t r i c t e c h n i q u e s b e c a u s e of the small quantities i n v o l v e d a n d the n a t u r e of the c o m p l e x . P r e v i o u s w o r k e r s e s t i m a t e d the a m o u n t of c h l o r i d e in the w a t e r - l e a c h e d s a m p l e s a n d found that w a t e r extracte d all c h l o r i d e ions. This was t a k e n as e v i d e n c e that no c o m p l e x of CaC12 formed. The p r e s e n t w o r k has s h o w n that l e a c h i n g with water, in fact, d e c o m p o s e s this c o m p l e x , w h e r e a s alcohol r e m o v e s only f r e e CaCI., without interfering with the c h l o r i d e c o m p l e x , C3S o r Ca(OH).). Surface a d s o r p t i o n in the C3S-CaC12-H20 system, as a p r e l u d e to a c c e l e r a t i n g action, was i n v e s t i g a t e d b y a few m o r e e x p e r i m e n t s . Tricalcium silicate was h y d r a t e d in w a t e r for 3 h r while still in the soc a l l e d d o r m a n t p e r i o d . The s a m p l e was v a c u u m d r i e d a n d one p a r t h y d r a t e d in water, the o t h e r with 5 p e r cent CaC12. The results a r e s h o w n in figures 3 a n d 4. A c c e l e r a t i o n of the formation of Ca(OH)2 s e e m s to take p l a c e within 1 h r in water. This, t o g e t h e r with 3 h r of p r e h y d r a t i o n , is e q u i v a lent to the p e r i o d for a c c e l e r a t i o n if C~S is d i r e c t l y t r e a t e d with water. It m a y i n d i c a t e that in the d o r m a n t p e r i o d it is the state of the solid p h a s e that significantly c o n t r i b u t e s to the reaction. In the p r e s e n c e of 5 p e r c e n t CaCI~ the pretreated s a m p l e exhibits an e n d o t h e r m a l effect c o r r e s p o n d i n g to the surface c h l o r i d e c o m p l e x for 1 hr. At 2 h r acceleration of h y d r a t i o n is e v i d e n t . In C~S d i r e c t l y e x p o s e d to 5 p e r cent CaCI~ the d o r m a n t p e r i o d is 2 h r a n d a s u r f a c e c o m p l e x exists b e f o r e a c c e l e r a tion (fig. 5). T h e s e r e s u l t s confirm that a surface c o m p l e x forms at a n y s t a g e d u r i n g the d o r m a n t period a n d is a p r e l u d e to the a c c e l e r a t i n g stage.

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30 t'AIN

.l MR

PREHYDRATED

2HR

IHR 30 M I N

3HR j.-

2HR 1HR

o~E i.-

4 HR

2HR

t-

4HR

4HR z

I DAY

7 DAYS

I 200

I 400

600

800': C

TEMPERATURE

I

I

I

200

400

600

IO0~C

TEMPERATURE

Fig 3. ~ Hydration behaviour of pre-hydrated 3CaO. SiO, with 5 ~ b CaCI2,

-

Cherrtisorbed Chloride Layer on the Surface C-S-H and Chloride in the Interlayer Space

of

The e m e r g e n c e of an i n t e n s e e x o t h e r m i c p e a k in the DTA c u r v e of C3S a l w a y s c o i n c i d e s with the onset of a c c e l e r a t i o n d u r i n g h y d r a t i o n in the p r e s e n c e of v a r y i n g amounts of CaCI~ (fig. 5). It was first thought that this c o u l d b e d u e to crystallization of the d e h y d r a t e d C-S-H to ~-wollastonite o r ~-C~S. The CSH (I) p r o d u c t is k n o w n to g i v e an e x o t h e r m i c p e a k of l a r g e m a g n i t u d e , but this occurs at t e m p e r a t u r e s b e y o n d 800 ~ T o b e r m o r i t e gel, o r CSH (II), s h o w s only v e r y small e x o t h e r m a l dents at t e m p e r a t u r e s b e y o n d 850 ~ F u r t h e r e x p e r i m e n t s i n d i c a t e d that it is v e r y u n l i k e l y the e x o t h e r m a l effect is only a crystallization effect of d e h y d r a t e d CSH (I) o r CSH (II), S a m p l e s of tricalcium silicate w e r e h y d r a t e d in w a t e r o r in 5 p e r cent CaCI~ a n d the resultant p r o d u c t s w a s h e d with w a t e r o r alcohol. F i g u r e 5 r e f e r s to C3S h y d r a t e d with 5 p e r cent CaC12 for different lengths of time a n d w a s h e d with a b s o l u t e alcohol. F i g u r e 6 r e p r e s e n t s the t h e r m a l b e h a v i o u r of s a m p l e s w a s h e d with water. A b l a n k e x p e r i m e n t was also c o n d u c t e d b y h y d r a t i n g C~S without CaC12 for different p e r i o d s of time a n d s u b s e q u e n t l y w a s h i n g e a c h with e x c e s s w a t e r (figl 7). This set of c u r v e s was o b t a i n e d at a sensitivity different from those r e p o r t e d e a r l i e r a n d cannot b e d i r e c t l y c o m p a r e d . A s u d d e n a c c e l e r a tion effect a n d the e m e r g e n c e of the e x o t h e r m i c effect at 2 h r was, h o w e v e r , o b s e r v e d in C3S h y d r a t e d in 5 p e r cent CaCI 2 (fig. 5). W a s h i n g with a b s o l u t e alcohol has no effect on e i t h e r the e x o t h e r m a l effect o r the Ca(OH)2 p e a k . S a m p l e s of h y d r a t e d C3S not t r e a t e d with alcohol w e r e identical to those r e p o r t e d in f i g u r e 5 a n d a r e not s h o w n s e p a r a t e l y . W a s h i n g with w a t e r e l i m i n a t e d the e x o t h e r m i c p e a k in all s a m p l e s (fig. 6). The b l a n k runs of s a m p l e s of C3S h y d r a t e d in w a t e r for different lengths of time

200

400

600

800 9OI) C

TEMPERATURE

Fig. 4. Hydration of 3 CaO. SiO, prehydrated for 3 hours. -

0

Fig. 5. - - Effect of leaching with alcohol on the exothermai behaviour of 3 CaO. SiO~ hydrated in presence of 5 ~b CaCI~.

a n d w a s h e d with e x c e s s of w a t e r d i d not e x h i b i t a n y s p u r i o u s effect that c o u l d i n t e r f e r e with o r annul the e x o t h e r m a l effect (fig. 7). The s a m p l e s d e s c r i b e d in figures 5 a n d 6, l e a c h e d with a b s o l u t e alcohol o r w a t e r , w e r e a n a l y s e d for c h l o r i d e content. By k n o w i n g the total c h l o r i d e cont e n t in the s a m p l e b e f o r e e x t r a c t i o n a n d that p r e s e n t in the extract the p e r c e n t a g e of u n e x t r a c t a b l e chlor i d e c o u l d b e calculated. F i g u r e 2 g i v e s the r e l a tive e x t r a c t i o n effects of a l c o h o l a n d w a t e r . At 2 hr, d u r i n g which p e r i o d the r e a c t i o n is a l r e a d y a c c e l e r a t e d , all the c h l o r i d e is e x t r a c t e d b y water, w h e r e a s about 56 p e r cent of the c h l o r i d e is u n e x t r a c t e d b y alcohol. At 4 hr, h o w e v e r , e v e n with water, 14 p e r cent c h l o r i d e is u n e x t r a c t e d a n d with a l c o h o l the value i n c r e a s e s to 87 p e r cent. At 24 h r a n d 168 h r alcohol extracts n e g l i g i b l e a m o u n t s of c h l o r i d e . At 168 h r w a t e r can extract o n l y 78 p e r c e n t of the c h l o r i d e , e v e n with e x c e s s of w a t e r . T h e s e results m a y m e a n that t h e r e is less CaC12 in the f r e e state as h y d r a t i o n p r o c e e d s . Within 4 h r a m a j o r p r o p o r t i o n of c h l o r i d e m a y b e s t r o n g l y c h e m i s o r b e d b y the C-S-H p r o d u c t a n d h e n c e not b e r e m o v a b l e b y alcohol l e a c h i n g . It is c a l c u l a t e d that freshly f o r m e d C-S-H in CaCI~ has a l a r g e s u r f a c e a r e a of o v e r 200 m s / g a n d has b o t h electrostatic a n d v a n d e r W a a l ' s forces. T h e r e is e v i d e n c e that the C-S-H has a p o s i t i v e - c h a r g e d surface [37], a n d this s h o u l d e n c o u r a g e C1- ions to b e a v i d l y a d s o r b e d . The e x o t h e r m i c p e a k in the a c c e l e r a t o r y p e r i o d m a y r e p r e s e n t s o m e sort of i n t e r a c t i o n of the c h l o r i d e ions on the C-S-H surface. The e m e r g e n c e of this p e a k c o i n c i d e s with a c c e l e r a t i o n a n d f o r m a t i o n of a h i g h surface a r e a C-S-H p r o d u c t . It m a y b e r e a s o n e d that C,~H~OH d o e s not e x t r a c t f r e e CaC1,., e v e n if it is p r e s e n t in l a r g e quantities; b e i n g l a r g e r than the H20 m o l e c u l e , it cannot p e n e trate all the p o r e s in the C-S-H p h a s e . It is quite

V. S. R A M A C H A N D R A N 15 MIN

"5 M HR

IHR 2HR

HR 3HR 4HR HR HR DAY

|

=<

w

:.J

DAYS

1 DAY

Er-

,.= u.

7 DAYS

II

If, 200 0

200

400

600

800~

400

O

600

-

-

Fig. 8.

-

p r o b a b l e that s o m e CaC12 in the f r e e state m a y b e i n a c c e s s i b l e to C~HsOH. C o n s i d e r a b l e quantities, h o w e v e r , a r e c h e m i s o r b e d on the C-S-H surface. F o r e x a m p l e , specific s u r f a c e a r e a s of h y d r a t e d p o r t l a n d c e m e n t c a l c u l a t e d from H20, N 2, CHaOH, CaH7OH a n d CGH12 a d s o r p t i o n (using m o l e c u l a r a r e a s of 11.4, 16.2, 18.1, 27.7 a n d 39 A s, r e s p e c t i v e l y ) a r e 194.6, 97.3, 88.5, 49.0 a n d 48.0 m2/g, r e s p e c t i v e l y [38]. The m o l e c u l a r a r e a of C~HsOH is m o r e than that for CH3Ot-t but less than that for CaH7OH; it is r e a s o n a b l e to e x p e c t a b o u t 30 to 40 p e r cent of the s u r f a c e to b e a c c e s s i b l e to C2HsOH, b u t h y d r a t e d CaS c u r e d for 7 d a y s s h o w e d that C2HsOH r e m o v e s v e r y little chloride. This s h o u l d confirm that m o s t of the c h l o r i d e ions a r e c h e m i s o r b e d on the h y d r a t e d CaS (or in a state not f r e e l y r e m o v a b l e with C2HsOH ). The a b o v e a r g u m e n t is b a s e d on the p r e m i s e that the s u r f a c e area, with H20, r e p r e s e n t s the c o r r e c t figure. T h e r e is s t r o n g e v i d e n c e , h o w e v e r , that the s u r f a c e a r e a b y N~ a d s o r p t i o n is in fact the t r u e figure. If so, t h e r e is s t r o n g e r e v i d e n c e that C1- m a y be chemisorbed. T h e r e is e v e r y p o s s i b i l i t y that the influence of CaC12 on h y d r a t i n g CaS c r e a t e s conditions u n d e r which c h l o r i d e ions m a y also exist in the i n t e r l a y e r s p a c e of the C-S-H p r o d u c t . T h e s e c h l o r i d e ions m a y b e unaffected b y C2HsOH, w h e r e a s H20, b e i n g s m a l l e r in d i a m e t e r a n d with h i g h e r d i p o l e m o m e n t is c a p a b l e of e x t r a c t i n g t h e m from the i n t e r l a y e r e v e n t h o u g h the s a m p l e s a r e d r i e d p r i o r to leaching. F e l d m a n a n d S e r e d a [39, 40] h a v e d e m o n s t r a t e d , b y m e a n s of s c a n n i n g i s o t h e r m s , a n d F e l d m a n , b y r e c e n t investigation of h e l i u m diffusion into c e m e n t paste, that w a t e r e n t e r s the i n t e r l a y e r s p a c e s e v e n at low humidities. The intense e x o t h e r m i c p e a k o b t a i n e d in h y d r a ting CaS in the p r e s e n c e of CaC12 can also b e r e p r o d u c e d b y t r e a t i n g c o m p l e t e l y h y d r a t e d C~S with a

I 600

I

I

800 900~ -

-

Thermal behaviour of hydrated 3CaO. SiO~ treated with CaCI~. (1) C3S hydrated 8 months

Fig. 7. Effect of leaching with water on the thermograms of 3Ca. OSiO~ hydrated in water to different periods. -

I 400

TEMPERATURE

800~

TEMPERATURE

TEMPERATURE

Fig. 6. Effect of leaching with water on the exothermal behaviour of 3CaO. SiO= hydrated in presence of 5 ~ o CaCI~.

I 200

- -

I treated with 5 (~o CaC1._, 2 extracted with alcohol 2 extracted with water I treated with t ~o CaC12 C3S hydrated for 6 hours -I (~) CaCl2 (7) C~S hydrated for 6 hours. (2) (3) (4) (5) (6)

weak solution of CaCl 2. Figure 8 gives the D T A curves of CaS hydrated for 8 months before and after treatment with 1 per cent or 5 per cent CaC12 (fig.8, curves i, 2 and S). Exothermic peaks are evident in both samples, followed by typical endothermal dips. Washing with alcohol has no effect on the exothermic peak, whereas washing with water removes it (fig. 8, curves 3 and 4). Even a 30 rain contact of the CaCl 2 solution with completely hydrated C3S is sufficient to produce this exothermic peak. In such a short period and with low concentrations of CaCI 2 no drastic structural changes in the C-S-H phase could be expected. The exothermic peak can also be generated at any stage of hydration of CaS. A n example is given for C3S hydrated for 6 hr and treated with 1 per cent CaCl 2 (fig. 8, curves 6 and ?). The exothermal peaks occur at higher temperatures with 1 per cent CaC12 and this is also observed in hydrating GaS containing I per cent CaCI 2. As stated before, chemisorption of chloride on the C-S-H surface plus its presence in the interlayer spaces and subsequent interaction during heating m a y be responsible for this peak. That the exothermic peak is not just a solid-solid interaction between CaCI 2 and C-S-H was checked b y c a r r y i n g out DTA on a m i x t u r e of p o w d e r e d CaC12 a n d C3S p r e h y d r a t e d for 8 months. No e x o t h e r m i c p e a k r e s u l t e d , i n d i c a t i n g that a d d i t i o n of w a t e r in the C3S + CaC12 s y s t e m is e s s e n t i a l for the p r o d u c t i o n of the e x o t h e r m i c p e a k . It was of i n t e r e s t to i n v e s t i g a t e w h e t h e r the e x o t h e r m i c p e a k was a r e s u l t of o x i d a t i o n effects in the system. S a m p l e s of C3S h y d r a t e d in 4 p e r cent CaC1 a for 4 o r 14 d a y s w e r e s u b j e c t e d to continuousv a c u u m DTA. The r e s u l t s s h o w that the e x o t h e r m i c p e a k is e l i m i n a t e d (fig. 9). One m i g h t c o n c l u d e that o x i d a t i o n was i n v o l v e d in the evolution of this e x o t h e r m i c p e a k , but w h e n the s a m p l e s w e r e s u b j e c t -

VOL.

4 --

N'

19 --

1971

--

MATERIAUX

ET

CONSTRUCTIONS

e d to DTA in an N 2 a t m o s p h e r e the exotherms p e r sisted. Elimination of the e x e t h e r m i c peak in continuous v a c u u m was in fact not real a n d s e e m s to have b e e n a m a s k i n g action of the e n d o t h e r m a l effect. C o n t i n u o u s v a c u u m m a y d e c r e a s e the t e m p e r a t u r e of h i g h - t e m p e r a t u r e e n d o t h e r m a l effect b y m o r e than 150 ~ [41]. This o b s e r v a t i o n has an important implication in v a c u u m DTA studies so far r e p o r t ed.

I

i

~ L

I

% <

~J

I

4 DAYS-AIR 4 DAYS-VAC.

4 DAYSNITROGEN

.~

14

Z

DAYS-VAC.

14 DAYS-AIR

tm

=1 0

I

I

I

200

400

600

I 800 900'C

TEMPERATURE 14 DAYSNITROGEN

0

200

400

600

800

C

TE:~IPERATU RE

Fig. 9.

-

-

Fig. 10. - - Effect of leaching on tile exothermai characteristics of 3CaO. Si02 hydrated in presence of CaCI~. (1) C:,S + t ~o CaCl~ hydrated 6 hours (2) C:3S § I o~ CaCI~ 8 hours (alcohol leached) (3) t leached with water (4) 2 leached with water

Thermal behaviour of 3CaO. SiO~ hydrated in presence of CaCI_. : effect of vacuum or nitrogen.

The C3S samples h y d r a t e d in i p e r cent CaC12 are different from those h y d r a t e d with h i g h e r CaC12 contents in that they exhibit two e x o t h e r m a l peaks. O n e is attributed to the c h e m i s o r b e d i n t e r l a y e r c h l o r i d e on the C-S-H p r o d u c e , a n d the other to the crystallization of the d e h y d r a t e d C-S-H. A c o m p l e tely h y d r a t e d C3S t r e a t e d with 1 p e r cent CaCtz fails to show m o r e than one e x o t h e r m i c effect. Samples h y d r a t e d for 6 or 8 hr a n d w a s h e d with alcohol do not influence either of these e x o t h e r m a l peaks, w h e r e a s water r e m o v e s only a s i n g l e e x o t h e r m in the s a m p l e s c u r e d for 8 hr (fig. 10). The s e c o n d e x o t h e r m i c p e a k s e e m s to b e r e t a i n e d but n o w occurs b e y o n d 800 ~ o w i n g to the crystallization effect. In h y d r a t i n g C3S containing CaCI 2 the e n d o t h e r m a l dip following the exothermal effect always coexists with the latter. Both are r e m o v e d b y w a s h i n g with water, but they are resistant to w a s h i n g with alcohol. T o g e t h e r these effects m a y r e p r e s e n t reactions i n v o l v i n g combination a n d decomposition. Incorporated

C1- i n t h e L a t t i c e o f C-S-H

In s a m p l e s h y d r a t e d for l o n g e r p e r i o d s significant a m o u n t s of chloride ions are not r e m o v e d b y leaching with water. T h e r e is e v e r y possibility that these c h l o r i d e ions are intimately associated in the C-S-H lattice, but the exact position a n d n a t u r e of the forces i n v o l v e d should await m o r e detailed analysis. The C-S-H is k n o w n to i n c o r p o r a t e S O 3 - - a n d to modify the m o r p h o l o g y . Similar effects a r e p o s s i b l e in the chloride t r e a t e d C-S-H products. In a r e c e n t p a p e r Richartz [42] found that p r o l o n g e d t r e a t m e n t of C3S with CaCI_~ at 80 ~ u n d e r autoclave conditions i n d i c a t e d some e n t r y of chloride ions into the lattice of C-S-H.

Role of CaCL in the Hydration

o f 3 C a O S i O 2.

Search for a p o s s i b l e c h l o r i d e c o m p l e x in the C3S-CaC12-H20 system has so far p r o v e d to b e of no avail. P r e s e n t data show that calcium c h l o r i d e m a y exist in four or five forms, i n c l u d i n g c o m p l e x e s , d u r i n g the h y d r a t i o n of C3S, the relative a m o u n t s d e p e n d i n g on how far the h y d r a t i o n has p r o g r e s s e d a n d on the c o n c e n t r a t i o n of CaCL. Especially d u r i n g the induction p e r i o d , it is p r e s e n t m a i n l y as free calcium chloride. As s o o n as the CaCL solution c o m e s into contact with the C:{S surface, some of it is avidly a d s o r b e d . In the a c c e l e r a t o r y stage a n d later it is b o u n d as a c h e m i s o r b e d l a y e r on the C-S-H surface a n d m a y exist in the interlayer. At later p e r i o d s the chloride also is firmly i n c o r p o r a t e d in the C-S-H phase, but the exact forces a n d position a r e not yet clear. T h e r e is g e n e r a l a g r e e m e n t that as soon as C3S comes into contact with w a t e r the first p r o d u c t f o r m e d d u r i n g the d o r m a n t p e r i o d is a coating with a C a O / SiO 2 ratio of n e a r l y 3 [43 to 47]. In the a c c e l e r a t o r y p e r i o d the CaO/SiO 2 ratio of the C-S-H p r o d u c t is m u c h l o w e r than 3: At this s t a g e the i n c r e a s e d rate of reaction m a y b e d u e to o n e or m o r e of the followi n g effects : autecatalytic effect, splitting off the layer, n u c l e a t i n g effect or formation of reaction centres, i n c r e a s e in the p e r m e a b i l i t y of the layer, etc. It is p o s s i b l e that the rate of formation of the initial l a y e r of high CaO/SiO~ ratio, its c o n v e r s i o n to a hydrate with l o w e r CaO/SiO.~ ratio a n d ultimate conv e r s i o n to hydrate, p o s s i b l y with a slightly h i g h e r C a O / S i O z ratio than the s e c o n d , a r e r e f l e c t e d as c h a n g e s in induction p e r i o d , setting time, s u r f a c e area, rate of hydration, m i c r o s t r u c t u r e , s h r i n k a g e a n d s t r e n g t h (table I). The t y p e a n d rate of inter-

V. S. R A M A C H A N D R A N

c o n v e r s i o n m a y b e dictated to a l a r g e extent b y the n a t u r e of the surface of the silicate p h a s e at v a r i o u s stages of hydration, a n d this in turn m a y d e p e n d on e n v i r o n m e n t a l conditions. TABLE I RELATIVE PROPERTIES OF C3S HYDRATED tN H20 OR CACL SOLUTIONS Properties

C3S § 0 % CaClz

C~S ! 1% CaCI~

C3S § 4 % CaCL~

790 min 12.40 3-4 hr

525 min 11.95 3-4 hr

105 rain (2 o~ CaCI~) 11.55 about 3 hra; bout 2 hr (5 % CaCl2)

about !4 hr

about 9 hr

about 6 hr

1. Setting time 1C3S: ICoS mixture (8) . . . . . . 2. pH at 4 hr . . . . . . . . . . . . . . . . 3. Induction period by Ca(OH) 2 estimation . . . . 4. a. Period required to attain max rate of heat evolution (w/s = 1.0) . . . . . . . . . . . b. Heat eyol.ved at the above period, approx.

1.5 • 10-3 CaI S}c lg-1 3 • 10-3 CaI Sec-lg-1 5~7 x 10-3 CaI Sec-lg-1

(14) . . .~ .................. 5. Degree of'hydration by Ca (OH)2 estimation* 6 hr . . . . . . . . . . . . . . . . . . . .

30 days . . . . . . . . 6. Degree of hydration in 6 hr . . . . . . . . . 30 days . . . . . . . . 7. Compressive strength at

~3 ~2

. . . . . . . . . . terms of C3S reacted* . . . . . . . . . . . . . . . . . . . . 28 days Kg/cm ~ (3). "/

5 2 r i

~3

~2 ~2

190

310

/

8. Surface area of C-S-H product hydrated for 30 I days (N2). . . . . . . . . . . . . . . . . . 9. CaO/SiO 2 ratio of C-S-H at 28 days (3) . . . . . ] 10. Morphology of C-S-H at 30 days . . . . . . .

24.8 2.0 Needle-like

32.7 1.97 Platy and crinkled foils ~

51 53 {Z1

25O (3 o/,. CaC12) 69.92 2.16 Platy

* Degree of hydration is qualitatively represented by ~1, %, %, where ~1 > :~" > %.

It is e v i d e n t that on i m m e d i a t e contact of the C3S surface with CaC12 solution t h e r e should b e an interf e r e n c e a n d e v e n alteration of the type of the surface l a y e r f o r m e d otherwise. The i m p o I t a n c e of surface in the h y d r a t i o n of C3S in the p r e s e n c e of r e t a r d i n g a d m i x t u r e s has b e e n r e c o g n i z e d . In the first few hours, a d s o r p t i o n of c h l o r i d e ions modifies the ratio of C a O / S i O 2 of the h y d r a t e to a l o w e r value, c o m p a r e d with that f o r m e d without CaC12 [15]. The a d s o r p t i o n of chloride m a y also modify one or m o r e factors, viz., p e r m e a b i l i t y , dispersibility, a d h e s i v e force of the initial l a y e r to the C3S surface, a n d the n u c l e a t i n g or reaction centre. For example, CaC12 on silica gel has b e e n r e p o r t e d to d e c r e a s e the p e r m e a b i l i t y of the surface [48]. Reduction of the i n d u c t i o n p e r i o d at h i g h e r c o n c e n trations of CaCIz a n d e a r l y setting d e p e n d on t h e s e factors. In the acceleratory p e r i o d it is also p o s s i b l e that Ca(OH)2 which e n v e l o p s the C3S surface is r e m o v e d b y interaction with CaC12. At the s a m e time, chloride ions are continuously a d s o r b e d on the C-S-H phase, a n d s u b s e q u e n t l y in the interlayers, a n d t h e s e in t u r n influence the rate of c o n v e r s i o n a n d n u m b e r a n d type of layers of C-S-H formed, a n d s u b s e q u e n t l y , their m o r p h o l o g y a n d specific surface. Ultimate s t r e n g t h is not d e p e n d e n t solely on the d e g r e e of hydration, but on the type of C-S-H f o r m e d a n d the a m o u n t of CaC12 intimately associated with C-S-H. F o r example, a h i g h e r CaO/SiO 2 p r o d u c t f o r m e d in the p r e s e n c e of 4 or 5 p e r cent CaCI 2 has m o r e i n c o r p o r a t e d chloride ions, a n d this m a y b e a factor m m a k i n g the resultant p r o d u c t weak c o m p a r e d with C3S h y d r a t e d with l o w e r CaC12 c o n c e n t r a t i o n (table I). H i g h e r c o m p r e s s i v e strengths in the C~S-CaC12H,,O system n e e d not b e d u e to the C-S-H p r o d u c t s

b e i n g of h i g h e r area, as has b e e n a s s u m e d b y Celani et al [10]. Surface area results u s i n g N 2 as a d s o r b a t e gives values for C-S-H p r o d u c t at 30 days e q u i v a l e n t to 24.8, 32.7 a n d 69.92 m2/g for C3S + 0 p e r cent CaCI~, C3S § 2 p e r cent CaCI~ a n d C3S § 4 p e r cent CaC12, respectively. Although C3S with 4 p e r cent CaC12 shows highest surface area, this s a m p l e shows lowest mechanical strength, indicating that the n a t u r e of the C-S-H p r o d u c t a n d CaO/SiO 2 ratio h a v e to b e taken into account in establishing a relation b e t w e e n s t r e n g t h a n d other p r o p e r t i e s a n d surface areas. The h i g h e r strengths with 1 or 2 p e r cent CaC12 should m e a n that, u n d e r these conditions, C-S-H p r o d u c e d has a l o w e r C a O / S i O 2 ratio p r o d u c t than that with 4 p e r cent CaCI 2 a n d also a high surface area. Jn addition, the m i c r o s t r u c t u r e m a y play a n important role in the d e v e l o p m e n t of strength. A c o m p a r i s o n of the e l e c t r o m i c r o g r a p h s of C3S h y d r a t e d for 30 days with 0, 1 or 4 p e r cent CaC12 a n d disp e r s e d in alcohol shows the p r e s e n c e of small n e e d l e s in C3S h y d r a t e d with water, w h e r e a s that h y d r a t e d with CaC12 s h o w e d platy or c r u m p l e d foil-like structure p r e d o m i n a t i n g (fig. 11). C o l l e p a r d i [49] also has o b s e r v e d that CaC12 stabilizes the platy structure. The c h e m i s o r p t i o n of chloride on the C-S-H surface m a y b e r e s p o n s i b l e for the c h a n g e s in m o r p h o l o g y . The chloride ions i n c o r p o r a t e d into C-S-H are not e x p e c t e d to b e m o b i l e e n o u g h in w a t e r solution to cause corrosion in r e i n f o r c e d systems. In e s s e n c e the reaction of C3S with water in the p r e s e n c e of CaCI 2 is v e r y complex. It is to b e r e c o g n i z e d that a d s o r p tion, substitution, a n d solubility m a y all play significant roles to different d e g r e e s , d e p e n d i n g on the reactants, e x p e r i m e n t a l conditions, a n d d u r a t i o n of hydration. These, in turn, influence the physical, chemical a n d mechanical p r o p e r t i e s of the products.

VOL.

4 --

N '~ 1 9

--

1971

--

MATI~RIAUX

ET

CONSTRUCTIONS

(b) CaS + i o~ CaCl2

(a) C3S ,2_ 0 o~ Cat12

Fig. 11. - - Electron micrographs of tricalcium silicate hydrated for one month (mag: • 12, 000).

CONCLUSIONS

(c) C3S -[- 4 o~)/ CaCI2

Calcium c h l o r i d e m a y exist in different forms in h y d r a t i n g tricalcium silicate, d e p e n d i n g on the initial mix p r o p o r t i o n s a n d d u r a t i o n of h y d r a t i o n . T h e s e a r e (i) free calcium c h l o r i d e , (ii) a c o m p l e x on the surface of C3S d u r i n g the d o r m a n t p e r i o d , (iii) a c h e m i s o r b e d l a y e r on the h y d r a t e d calcium silicate, (iv) i n t e r l a y e r chloride, a n d (v) c h l o r i d e intimately b o u n d in the lattice.

ACKNOWLEDGEMENT

Thanks a r e d u e to P.J. S e r e d a a n d R.F. F e l d m a n for helpful discussions a n d to G.M. P o l o m a r k a n d E.G. Quinn for e x p e r i m e n t a l assistance. This p a p e r is a c o n t r i b u t i o n from the Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, a n d is p u b l i s h e d with the a p p r o v a l of the D i r e c t o r of the Division.

10

V. S. R A H A C H A N D R A N

R~SUM~ Etats possibles du chlorure au cours de l'hydratation du silicate tricalcique en prdsence de chlorure de calcium. ~ L'hydratation du silicate tricalcique en prdsence de chlorure de calcium s'accompagne de rdactions endo et exothermiques qu'on n' observe pas dans d'autres circonstances. La rdaction endothermique, qui se produit entre 550 et 590 ~ est attribude ~ la formation d'une couche de chlorure la surface du silicate lors de l'avant-prise. Une intense rdaction exothermique, apparaissant entre 640 et 690 ~ co)ncide avec une p~riode d'hydratation accdldrde et est attribude ~ la sorptioncombinaison de chlorure sur le silicate et ~ la prdsence de chlorure dans les couches de structure. On peut obtenir cette rdaction exothermique en faisant agir CaC12 sur le silicate tricalcique ~ tout moment de l'hydratation. L'analyse thermique diffdrentielle continue sous vide p e r m e t d'dliminer le pic exothermique, exceptd lorsque l'expdrience se fait dans un courant d'azote. L'endotherme obtenu durant l'avant-prise et le pic exothermique formd lors de la pdriode d'accdldration p e u v e n t ~tre ~liminds par Iavage ~ l'eau des dprouvettes. On peut

extraire ~ l'alcooi environ 13 0,o du chlorure ajout6 durant quatre heures d'hydratation, mais apr~s sept jours, le chlorure n'est pratiquement plus extrait. Les valeurs correspondantes pour l'extraction l'eau sont de 86 et de 78 ~ I1 est supposd que Ie chlorure de calcium existe sous quatre ou cinq formes diffdrentes, m d m e complexes, lors de l'hydratation du silicate tricalcique, selon sa proportion et la durde de l'hydratation. I1 y a prdsence de CaCI 2 libre dans les p r e m i e r s temps de l'hydratation-. Durant I'avant-prise, le chlorure est adsorbd aussi ~ la surface du silicate tricalcique. Au cours de la pdriode d' accdldration, et apr~s le chlorure est adsorbd sur Ies silicates hydraMs produits, et en partie sur les couches de structure. Ultdrieurement une quantitd importante de chlorure s'incorpore intimement aux formations de silicates hydraMs et ne peut ~tre extraite ~ l'eau. En [onction de la durde d'hydratation et des formes diverses de chlorure, iI est possible qu'une action s'exerce sur : l'avant-prise, le temps de prise, l'accdldration, la CaO surface ddveloppde, le retrait, le rapport s~-o~~ du silicate hydraM produit, la morphologie et Ia rdsistance.

REFERENCES

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